Multimeter Guide

What is a multimeter?

A multimeter is a versatile device that can be used to accurately measure different electrical values in components and circuits. A multimeter can be used to measure current, voltage, resistance and capacitance. They can also perform a continuity test (used to detect broken circuits) and a diode test.

Main Measurement Functions:

• Voltage DC (Volts, V): Measures the voltage between any two points in an AC or DC circuit.
• Current DC (Amperes, A): Indicates the rate of flow of electrical current through a circuit or component, usually 10A maximum. For higher values, use a clamp meter.
• Resistance (Ohms, Ω): Measures how strongly a component resists the flow of current.
• Continuity:Makes an audible noise when a circuit is complete (closed). If the circuit is broken (open) the multimeter will not make a noise.
• Capacitance (Farads, F): Measures the capacitance of a component or circuit.

Common uses:

Troubleshooting: Verifying that the voltage and current are within the expected range to help locate electrical faults in vehicles, 12V systems and electronic devices.
Component Testing: Verifying that components like switches, resistors and fuses are operating correctly.

How do I use a multimeter to test a battery’s voltage? (step by step)

Safety First!

Before you begin, it's crucial to follow a few safety precautions. Car batteries contain corrosive acid and can produce flammable gases.

• Turn everything off: Make sure your car's engine, headlights, radio, and all other electronics are off (unless you are specifically testing things like the charging voltage which require the engine to be running).
• Wear your gear: Put on your safety glasses and gloves (do not wear gloves near a running engine).
• Ensure good ventilation: Work in a well-ventilated area, like an open garage or outdoors.
• Keep metal away: Be careful not to let any metal tools touch both battery terminals at the same time, as this will cause a short circuit. Ensure any spanners, screwdrivers and ratchets are put away when they are not being used. Ensure you remove any watches and jewellery as they are often metallic and could result in a burn injury if involved in a short circuit.

Step 1: Locate the battery

Ensure that you can access the two terminals on top of the battery. Most batteries are in the engine bay however they can also be located in the boot or under a seat. Check the vehicle user manual or call your local R&J branch for more help locating the battery. You may be required to remove a plastic cover or some trim pieces to access the battery.

Step 2: Ensure the vehicle is switched off:

For an accurate reading of the battery voltage, ensure the vehicle is switched off. You may need to isolate the battery from the system by disconnecting the positive or negative terminal.

Step 3: Connect the Multimeter:

Turn the multimeter dial or settings to DC Voltage position. Connect the positive probe to the positive terminal and the negative probe to the negative terminal; you’ll see the terminals marked with '+' and '-'.

Step 4: Read the voltage:

Observe the reading on the multimeters display.

If you see a reading with a minus sign in front (-12.6V instead of 12.6V), that means the probes are reversed. Ideally, the resting voltage of a charged, healthy lead acid battery should be no lower than 12.6V.

A fully charged lead acid car battery should have a healthy voltage. Here’s what the numbers mean:

• 12.6V or above: Your battery is fully charged and in good shape.
• 12.1V to 12.5V: Your battery is partially discharged and should be charged ASAP.
• Less than 12.2V: Your battery is severely discharged and should be charged as soon as possible. You should use a multistage battery charger for this. The battery may also have a fault, or it may be at the end of its service life. You should get the battery checked by a professional as soon as possible.

Voltage Drop Under Load (cranking)

A weak battery can struggle to maintain voltage when under load (e.g. during engine cranking). Over time the internal components of a battery will degrade which can reduce the battery’s ability to store energy and deliver enough power to start the engine.

Using a multimeter to check your battery voltage may help you detect a change in the health of your battery. This can help avoid being surprised with a flat or failed battery. We recommend the REDMAX multimeter for easy testing. It is a Digital Pen Type Multimeter which is easy to use and carry.Learn more about the product here: https://www.rjbatt.com.au/redmax#multimeter

What’s the difference between a multimeter and a battery tester?

Both a multimeter and a battery tester can help you understand the condition of a battery. However. one of them is able to give you a much more comprehensive diagnosis of the battery. Here's a breakdown of the key differences:

1. Functionality

• Multimeter:A multimeter can measure voltage, which can be used to check the State of Charge of the battery, and also check the charging voltage when the engine is running.
• Electronic Battery Tester: This specialised tool not only measures the voltage, it will also determine the State of Health of the battery by measuring the internal resistance. It provides a more detailed assessment of the battery's health and will provide an overall diagnosis.

2. Ease of Use

• Multimeter: The multimeter is quick and easy to use. It’s handy for a quick battery voltage check, especially if you’re using a 2 handed Digital Pen type multimeter like the REDMAX Multimeter.
• Battery Tester:: A tester doesn’t take much longer than the multimeter to use however it provides more information. Battery testers have a digital display that provides clear, straightforward results, and will provide a diagnosis like Good Battery, Good Battery - Recharge, Charge & Re-Test or "Replace Battery."

3. Cost

• Multimeter: : Generally more affordable and widely available. It’s a great tool for DIY enthusiasts who need a multipurpose device.
• Electronic Battery Tester: More expensive due to its specialised features, however it is a necessary tool for those who frequently test batteries or work in automotive repair.

4. Output

• Multimeter: Displays raw data (e.g., voltage or resistance), leaving the interpretation up to the user.
• Electronic Battery Tester: Provides a detailed diagnosis which includes the overall condition of the battery , and whether it needs to be charged of replaced. Some testers are also able to test the charging system.